The present invention relates to cardiac ablation systems, and more particularly to RF ablation systems in which a radio frequency ablation catheter is inserted, typically via a cut down to a major artery or vein, to access the wall of the heart and "ablate" tissue by the application of RF energy thereto. The energy is applied at a level and for a time effective to denature or kill the active tissue and thereby eliminate a region of myocardium or abnormal electrical pathway responsible for an arrhythmia.
A number of techniques have been developed for identifying the relevant sites in the cardiac wall, and for guiding an ablation catheter to the correct site for treatment. In particular, there exist a number of so-called mapping catheters which are inserted to detect and map cardiac signals. These catheters typically have a plurality of electrodes which each are operated in a sensing mode to receive cardiac signals and identify the location of the target site, and have one or more electrode portions which may be actuated to apply energy and treat the site. A reasonable degree of accuracy is achieved by such mapping catheters. Nonetheless, treatment of cardiac sites in this manner remains subject to a number of uncertainties due in part to the inaccuracies of determining the exact lesion site, and in part to the difficulty of applying the correct amount of energy to dependably ablate the site. Ablation is effected by raising the tissue temperature sufficiently high for a suitable length of time. Typically, in current technology, one does not actually measure the tissue temperature achieved in the cardiac wall but only the temperature of the catheter tip or of the electrode portion contacting the wall. This is done, for example by using an ablation catheter such as the Medtronic RF Ablater, which has a temperature sensor centrally located in its electrode tip. Typically, the catheter control console is set to achieve a tip temperature of 70-80 degrees centigrade, with the assumption, based on experimental data, that actual tissue temperatures will be approximately 0-10 degrees centigrade higher. Nonetheless, even at these temperatures, it is not uncommon for conduction to return after a lapse of fifteen to twenty minutes, requiring a second round of targeting and ablation to be undertaken. In addition, ineffective applications which produce these high temperatures may kill areas of heart tissue unnecessarily, and may produce long term risks.
Thus, it would be desirable to develop a method which more accurately identifies a target site and positions an ablation electrode at the site.
If it would further be desirable to identify methods of ablation which are highly effective yet reduce the risk of damaging tissue sites at which block would not occur.